Because inductance value of inductor being an energy storage component in the buck converter will affect input current response speed and output current ripple, the input current response speed is faster and the output current ripple is relatively larger if the inductance value of inductor being an energy storage component is smaller; otherwise, the buck converter input current response speed is slower and the output current ripple is smaller if the inductance value of inductor being an energy storage component is larger. Therefore, the traditional buck converter often uses the inductor with smaller inductance value and the output capacitor with larger capacitance value in order to achieve a faster input current response speed and a lower output current ripple. However, the capacitor with large capacitance value is required to use electrolytic capacitor, which is susceptible to the switch and the temperature such external environment factors causing its short service life and thus shorten the service life of capacitor.
In traditional non-isolated buck converter, the ideal buck converter as shown in FIG. 1 is the most basic traditional buck converter which mainly composed of the control IC, switching components, diodes, energy storage inductor and capacitor. The operation theory is described as below: when the power transistor is conducted, the voltage source charges the energy storage inductor, the capacitor C and provides energy to the output load; when the power transistor is cut off, the energy storage inductor uses its stored energy via the diode D to charge the capacitor C, also provides energy to the output load.
FIG. 2 shows the current widely used traditional flyback converter, which mainly used in less then 100 W isolation type buck converter. Since the circuit is simple and low cost, the flyback transformer in the circuit can also be used as energy storage, and the secondary end only need a diode and a capacitor. From a cost point of view, the flyback converter circuit is very competitive in the market. The flyback converter circuit is mainly composed of a control IC, a power transistor, a flyback transformer, a diode D and a capacitor. The control IC is used to control the power transistor conduction or cutoff, via the flyback transformer magnetizing inductance to proceed the energy storing and releasing, and cooperating with a secondary side the diode and the capacitor to proceed the rectifying and filtering of the output voltage, and then to get the output of the DC voltage. Its basic operation principle as follows: Due to the flyback transformer T, the circuit has electrical isolation, transformer and energy storage inductance of the triple function. Rigorous theoretical terms, the flyback transformer is not a real transformer, but a coupling inductor. By controlling the power transistor conduction or cutoff, the energy storage in the flyback transformer is passed to the secondary side, via the diode charging to the capacitor C and maintains the DC voltage at the set value. When the power transistor is conduction, the voltage source Vin is charging to the flyback transformer, and causes the diode D reverse-bias, meanwhile the capacitor C provides energy to the output capacitor. When the power transistor is cutoff, the energy of the flyback transformer via the diode charging to capacitor C and provides energy to the output terminal.